An automatic transmission mounted in a vehicle is provided with a selector lever that is operated by a driver. And, a shift control device (a change-speed control device) controls shift (speed) of the automatic transmission according to a selection range selected by the selector lever.
The automatic transmission is provided with a range detector called an inhibitor switch which when the selector lever is operated to any range position of D, N, R, P etc., detects its selection range and outputs a selection range signal of the detected selection range.
With regard to the selector lever, a manual valve, which changes a shift state of the automatic transmission by changing a hydraulic pressure supply passage leading to the automatic transmission, is connected to the selector lever so as to mechanically work with or synchronize with the selector lever.
The shift control device controls the shift of the automatic transmission according to the selection range signal sent from the inhibitor switch. However, when the inhibitor switch is in an abnormal state, the shift control device controls the automatic transmission by a fail-safe control executed for this abnormal state. Thus, it is necessary to judge whether the inhibitor switch is in the abnormal state or in a normal state.
As the abnormality of the inhibitor switch, there is a no-signal abnormality (a signal-absence abnormality) that indicates that a detection signal is not outputted from the inhibitor switch. A main cause of this no-signal abnormality is poor contact (contact failure) of electric contact provided at each range position of D, N, R, P etc. and breakage of an electric signal line.
However, even if the poor contact or the breakage does not occur in the inhibitor switch, when the selector lever is positioned at a middle position (hereinafter, called a middle range position) between adjacent two range positions, the detection signal is not outputted from the inhibitor switch. Because of this, it is not always possible to judge, by the fact that the detection signal is not outputted from the inhibitor switch, that the inhibitor switch fails.
Under the circumstances, for instance, Patent Document 1 proposes a technique that judges that if a state in which the detection signal is not outputted from the inhibitor switch continues for a predetermined time, the inhibitor switch is in a state of the no-signal abnormality.
That is, a state in which the selector lever is positioned at the middle range position is a state in which the operation of the selector lever by the driver is in progress. And, this state continues only for a few time (a few seconds) (e.g. 1 to 2 sec.) in a normal operation. However, in a case where the driver stops the selector lever operation in the middle of the range selection operation, the state of the middle range position continues for a time that is longer than this few time.
However, since it is assumed that a duration time (or a continuation time) of this case also has a limitation (e.g. about a few tens of seconds), if a state in which the detection signal is not outputted from the inhibitor switch continues for a predetermined time determined based on this limitation, the technique judges that the inhibitor switch is in the no-signal abnormality state.
In the above technique, however, even in a case where the inhibitor switch is really in the no-signal abnormality state, the abnormality of the inhibitor switch is not determined (fixed or ascertained) until the predetermined time elapses. Because of this, execution of the fail-safe control is delayed for this predetermined time.
For this reason, it is desirable to shorten a time required to determine the no-signal abnormality of the inhibitor switch as much as possible. Especially when the vehicle travels, it is desirable to shorten the time required to determine the abnormality then to shift the control to the fail-safe control in as short a time as possible.